I would only apply such standard if it passes the symmetry principle with currently legal houses which is the context in which above mentioned about the not being worth bulldozing/fixing if they met the old codes. I.e. if the problem was legal for the construction of a currently inhabited house then the same logic about not being worth it to start over applies to the new house as the old house. Therefore it couldn't possibly devolve into codes being meaningless but rather worst case devolve into creating a baseline of allowing older codes which apparently are safe enough we haven't kicked their owners out. In effect it would create equality in home ownership rather than a standard where you can build a house and then vote to make it harder for the next guy to artificially raise your property values. If something is truly too unsafe to exist the rules should be applied uniformly rather than giving privileged immunities to early entrants.